This invention relates generally to a flexural plate wave sensor and array, and more particularly to a flexural plate wave sensor having reduced dimensions which enable the array to have an increased density of sensors on a single silicon wafer.
Flexural plate wave (FPW) devices are gravimetric sensors capable of detecting mass changes as small as 10xe2x88x9211 g. Typically, FPW devices are built with a bulk micro-machining process which produces a thin film membrane of silicon or silicon nitride by etching a cavity through the entire thickness of the silicon wafer with a selective process which does not attack the membrane material. However, due to the crystal structure of the silicon wafer, the cavity produced by this etching process has interior walls which extend through the silicon wafer at an angle of 126xc2x0 from the membrane. This results in the cavity having an opening at the bottom surface of the substrate which is at least twice as large as the area of the membrane. Accordingly, the smallest possible FPW device built utilizing the prior art bulk micromachining process is approximately 1 mmxc3x971 mm, since, for this one square millimeter of area on the surface of the silicon wafer, at least twice as much area is required on the bottom of the wafer. Therefore, only small numbers of FPW sensors can be integrated onto the same silicon chip for exposure to the same environment. For applications which require several sensors with different coatings, several packaged sensors must be integrated onto a sensor assembly and exposed to a gas or liquid sample stream. This method is only practical for applications requiring less than approximately 20 separate sensors.
It is therefore an object of this invention to provide a flexural plate wave sensor having reduced areal dimensions which enable an array of sensors to have an increased density of sensors on a single silicon wafer.
It is a further object of this invention to provide such a flexural plate wave sensor including a sensor membrane having increased sensitivity.
It is a further object of this invention to provide such a flexural plate wave sensor in which the sensor membrane is not sealed on one side, thereby eliminating atmospheric pressure variations in the membrane tension.
It is yet a further object of the invention to provide such a flexural plate wave sensor in which the absorptive coating is separate from the electrical components for sensing elements in fluid environments.
It is a yet further object of this invention to provide a flexural plate wave sensor array having increased packing density which enables a greater number of sensors to be fit on a single silicon chip.
It is a further object of this invention to provide a method of making a flexural plate wave sensor in which the sensor membrane is not exposed until the end of the manufacturing of the sensor.
This invention results from the realization that a truly effective flexural plate wave sensor can be obtained by bulk machining the silicon wafer to form a sensor having a cavity with substantially parallel interior walls, adding an etch stop layer and a membrane layer to the wafer, adding an absorptive coating on the membrane layer and transducers on the membrane layer opposite the absorptive coating. This construction facilitates the formation of an array of sensors having increased packing density on the silicon wafer.
This invention features a method for manufacturing a flexural plate wave sensor including the steps of depositing an etch-stop layer over a substrate, depositing a membrane layer over the etch stop layer, depositing a piezoelectric layer over the membrane layer, forming a first transducer on the piezoelectric layer and forming a second transducer on the piezoelectric layer, spaced from the first transducer. The method further includes the steps of etching a cavity through the substrate, the cavity having substantially parallel interior walls, removing the portion of the etch stop layer between the cavity and the membrane layer to expose a portion of the membrane layer, and depositing an absorptive coating on the exposed portion of the membrane layer.
In a preferred embodiment, the method may further include the steps of etching a hole in the piezoelectric layer and forming a ground contact on the silicon membrane layer.
This invention also features a flexural plate wave sensor including a base substrate, an etch stop layer disposed over the base substrate, a membrane layer disposed over the etch stop layer and a cavity having substantially parallel interior walls disposed in the base substrate and the etch stop layer, thereby exposing a portion of the membrane layer. The flexural plate wave sensor further includes an absorptive coating disposed on the exposed portion of the membrane layer within the cavity, a piezoelectric layer disposed over the membrane layer, a first transducer disposed on the piezoelectric layer, and a second transducer disposed on the piezoelectric layer, spaced from the first transducer.
In a preferred embodiment, the first and second transducers may be interdigitated transducers. The first and second transducers may be formed from TiPtAu or from aluminum. The piezoelectric layer may be formed from aluminum nitride, lead zirconium titanate or zinc oxide. The etch stop layer may be formed from silicon dioxide or from silicon and the base substrate may be formed from silicon.
This invention also features a method for manufacturing a flexural plate wave sensor including the steps of depositing a sacrificial material layer over a silicon substrate, depositing a membrane layer over the sacrificial material layer with the membrane layer covering the sacrificial material layer and contacting the silicon substrate and depositing a piezoelectric layer over the membrane layer. The method further includes forming a first transducer on the piezoelectric layer, forming a second transducer on the piezoelectric layer, spaced from the first transducer, removing the sacrificial material layer to expose a portion of the membrane layer and depositing an absorptive costing on the exposed portion of the membrane layer.
This invention also features a flexural plate wave sensor including a substrate, a membrane layer disposed on the substrate, the membrane layer having legs in contact with the substrate and a body portion spanning between the legs. The substrate, a lower surface of the body portion and interior surfaces of the legs define a cavity between the substrate and the body portion. The flexural plate wave sensor further includes an absorptive coating disposed on the lower surface of the body portion of the membrane layer, a piezoelectric layer disposed over an upper surface of the membrane material, a first transducer disposed on the piezoelectric layer and a second transducer disposed on the piezoelectric layer, spaced from the first transducer.
In a preferred embodiment, the substrate may be formed from silicon and the membrane layer may be formed from silicon. The first and second transducers may be interdigitated transducers that may be formed from TiPtAu or from aluminum. The piezoelectric layer may be formed from aluminum nitride, lead zirconium titanate or zinc oxide.
This invention also features a method for manufacturing a flexural plate wave sensor including the steps of depositing a membrane layer on a substrate having a concave upper surface, thereby forming a cavity between an exposed portion of the membrane layer and the substrate, depositing a piezoelectric layer on the membrane layer, forming a first transducer on the piezoelectric layer, forming a second transducer on the piezoelectric layer, spaced from the first transducer, and depositing an absorptive coating on the exposed portion of the membrane layer within the cavity.
This invention also features a flexural plate wave sensor including a substrate having a recess disposed in an upper surface thereof, a membrane layer disposed on the upper surface of the substrate, a cavity disposed between a portion of the membrane layer and the recess in the substrate and a piezoelectric layer disposed on the membrane layer. The flexural plate wave sensor further includes a first transducer disposed on the piezoelectric layer, a second transducer disposed on the piezoelectric layer, spaced from the first transducer and an absorptive coating disposed on the portion of the membrane layer within the cavity.
In a preferred embodiment, the substrate may be formed from a material selected from silicon or PYREX(copyright) material. The first and second transducers may be interdigitated transducers formed from TiPtAu or from aluminum. The piezoelectric layer may be formed from aluminum nitrate, lead zirconium titanate or zinc oxide.
This invention also features a flexural plate wave sensor array including a substrate, and a plurality of flexural plate wave sensors. Each sensor includes a cavity formed in the substrate, a thin film membrane layer spanning the cavity, a piezoelectric layer disposed on the thin film membrane layer, a transducer disposed on the piezoelectric layer and an absorptive coating disposed on the thin film membrane layer within the cavity. The cavity of each of the sensors includes interior walls that are substantially parallel to each other and to the interior walls of adjacent sensors.
This invention also features a flexural plate wave sensor array including a substrate and a plurality of flexural plate wave sensors. Each sensor includes a cavity formed in the substrate, a thin film membrane layer spanning the cavity, a piezoelectric layer disposed on the thin film membrane layer, a transducer disposed on the piezoelectric layer and an absorptive coating disposed on the thin film membrane layer within the cavity. The distance between adjacent sensors is no greater than 0.9 mm.
This invention also features a flexural plate wave sensor array including a substrate, a plurality of flexural plate wave sensors, each sensor including a cavity formed in the substrate, a thin film membrane layer spanning the cavity, a piezoelectric layer disposed on the thin film membrane layer, a transducer disposed on the piezoelectric layer and an absorptive coating disposed on the thin film membrane layer within said cavity; a reference flexural plate wave sensor including a cavity formed in the substrate, a thin film membrane layer spanning the cavity, a piezoelectric layer disposed on the thin film membrane layer and a transducer disposed on the piezoelectric layer; and a microprocessor electrically connected to each of the plurality of flexural plate wave sensors and the reference flexural plate wave sensor, for monitoring resonant frequency characteristics of the sensors.
In a preferred embodiment, the reference sensor may monitor the effects of environmental factors on the sensors and the microprocessor adjusts the resonant frequency of the sensors to compensate for the environmental factors.
This invention also features a flexural plate wave sensor array including a substrate and a plurality of flexural plate wave sensors, each sensor including a cavity formed in the substrate, a thin film membrane layer spanning the cavity, a piezoelectric layer disposed on the thin film membrane layer, a transducer disposed on the piezoelectric layer and a plurality of discrete absorptive coatings disposed on the thin film membrane layer within the cavity. The cavity of each of the sensors includes interior walls which are substantially parallel to each other and to the interior walls of adjacent sensors.
This invention also features a flexural plate wave sensor array including a substrate; a plurality of flexural plate wave sensors, each sensor including a cavity formed in the substrate, a thin film membrane layer spanning the cavity, a piezoelectric layer disposed on the thin film membrane layer, a transducer disposed on the piezoelectric layer and an absorptive coating disposed on the thin film membrane layer within the cavity; a drive amplifier which receives a drive input and outputs an amplified drive output; a multiplexer which receives the amplified drive output and a selection signal, for driving one of the plurality of flexural plate wave sensors; and an output amplifier which senses an output from the one of the plurality of flexural plate sensors and outputs an amplified sensed signal.